Sn-b plating solution and plating method using it

ABSTRACT

The object of the present invention is to prevent generation of whisker in a Pb-free plating layer. Provided is a Pb-free Sn—B plating solution containing tin sulfate, which is a source of Sn ions, and dimethyl amine borane or trimethyl amine borane, which is a source of B ions.

TECHNICAL FIELD

The present invention relates to a Sn—B plating solution without lead (hereinafter, referred to as a Pb-free Sn—B plating solution), and a plating method using the same, and more particularly, to a Pb-free Sn—B plating solution that can prevent generation of whisker in a plating layer, and a plating method using the same.

BACKGROUND ART

The present invention does not contain lead, and at the same time, prevents generation of whisker in a plating layer. A semiconductor lead frame has various shapes depending on high densification or integration of the semiconductor chip, or a method of mounting the semiconductor chip on a substrate.

Basically, the semiconductor lead frame is formed of a pad, in which a chip is mounted and maintains a static state of a chip thereon, i.e. a semiconductor memory device, an inner lead, which is connected to the chip by a wire bonding, and an outer lead, which connects the semiconductor lead frame to an external circuit. The semiconductor lead frame having such structure is conventionally manufactured using a stamping method or an etching method.

The semiconductor lead frame is packaged with the chip through assembling processes including a chip attaching process, a wire bonding process, a molding process, a marking process, a separating process, etc.

During such assembling processes, terminals of the pad and the inner lead are plated with a metal material, such as silver (Ag), in order to maintain a bondability of a lead wire that connects the chip and the inner lead and an excellent characteristic of the pad. Also, a predetermined region of the outer lead is plated with a solder material, i.e. tin-lead (Sn—Pb), in order to improve a soldering performance for substrate mounting after molding a resin protection film. However, it is difficult to perform such a plating method, and the semiconductor chip often malfunctions due to plating solution penetrating between the surface of the semiconductor lead frame and epoxy molding. Also, an additional process is required in order to remove un-uniformity of a plating layer.

Accordingly, a pre-plated frame (PPF) method is suggested, whereby a material having good lead wettability is pre-plated on the top surface of the semiconductor lead frame before the assembling processes. In the PPF method, a 2-layered structure, in which a nickel (Ni) layer is formed as an intermediate layer on a metal base material for a lead frame, such as copper (Cu), and a palladium (Pd) layer having good lead wettability is formed entirely or partially on the intermediate layer, a 3-layered structure, in which a Ni layer, a Pd layer, and a gold (Au) flash layer as the top layer are respectively formed on a base material, and a 4-layered structure, in which a Ni strike layer, a Pd—Ni alloy layer, a Ni layer, and a Pd layer are respectively formed on a base material, are commercially used. However, when the base material is Cu or an alloy, such as alloy 42 that does not include a Cu component, the semiconductor lead frame badly corrodes. Also, the price of Pd is unstable, and when the price of Pd increases, the manufacturing costs of the semiconductor package also increase.

Recently, a two-tone pre-plated frame method has been used, whereby a region corresponding to the inner lead and a region corresponding to the outer lead in the metal base material are independently plated with different metals. For example, the region corresponding to the inner lead may be plated with Ag, and the region corresponding to the outer lead may be plated with Sn—Pb.

However, a plating method used in the PPF method and the two-tone pre-plated frame method has several problems due to environmental contamination caused by lead. Throughout the world, various regulations are being put in place to control the use of lead in electronic products. Also, research is continually being conducted on a material that can replace solder paste using lead and a Sn—Pb plating material.

Pure Sn plating may be the best alternative for Sn—Pd plating. However, in pure Sn plating, short circuits may be formed due to excessive generation of whisker.

Whisker refers to protruding crystals that are generated on the surface of the plating layer when two different materials bond to each other and thus inter-diffuse. The whiskers are vulnerable to heat and humidity. When the whiskers are formed on the surface of the plating layer of a semiconductor lead frame, the semiconductor is electrically short circuited, and thus the circuit malfunctions.

In order to prevent the generation of whisker, a heat process after Sn plating, Ni plating, regulation of Sn particle size, and an alloy of Sn and a dissimilar metal are considered. A Sn—Bi (bismuth) alloy is widely used as the alloy of Sn and a dissimilar metal.

However, the Sn—Bi alloy cannot sufficiently suppress the generation of whisker, and the deposition potential difference between Sn and Bi is significant, and thus eutectoid is difficult. Also, when concentration of Bi in a solution is high, Bi is deposed on the surface of the cathode, and can fall off after soldering. Also, when content of Bi in a plating layer is high, cracks may form in the plating layer when the plating layer is bent.

DESCRIPTION OF THE DRAWINGS

FIGS. 1 (a) through (c) are scanning electron microscope (SEM) photographic images respectively illustrating surface states of plating layers in Experiments 1 through 3 after a normal temperature storage test;

FIGS. 2 (a) through (c) are SEM photographic images respectively illustrating surface states of plating layers in Experiments 4 through 6 after a room temperature storage test;

FIGS. 3 (a) through (c) are SEM photographic images respectively illustrating surface states of plating layers in Experiments 7 through 9 after a room temperature storage test;

FIGS. 4 (a) through (c) are SEM photographic images respectively illustrating surface states of plating layers in Experiments 10 through 12 after a room temperature storage test;

FIGS. 5 (a) through (c) are SEM photographic images respectively illustrating surface states of plating layers in Experiments 13 through 15 after a room temperature storage test;

FIG. 6 (a) is a SEM photographic image after plating the surface in Experiment 16, FIG. 6 (b) is a SEM photographic image of Experiment 16 after a room temperature storage test, and FIG. 6 (c) is a SEM photographic image illustrating surface states of a plating layer in Experiment 17 after a room temperature storage test; and

FIGS. 7 (a) through (e) are SEM photographic images respectively illustrating surface states of plating layers in Comparative Experiments 1 through 5 after a room temperature storage test.

DETAILED DESCRIPTION OF THE INVENTION Technical Problem

The present invention provides a Pb-free Sn—B plating solution that can prevent generation of whisker in a plating layer, and a plating method using the same.

Technical Solution

According to an aspect of the present invention, there is provided a Pb-free Sn—B plating solution containing tin sulfate, which is a source of Sn ions, and dimethyl amine borane or trimethyl amine borane, which is a source of B ions.

The amount of the source of Sn ions may be 15 to 50 g/L.

The amount of B ions may be 0.1 to 3.0 g/L.

The Pb-free Sn—B plating solution may further contain 30 to 70 ml/L of sulfuric acid. The Pb-free Sn—B plating solution may further contain 10 to 40 g/L of cresolsulfonic acid or phenolsulfonic acid. The Pb-free Sn—B plating solution may further contain 0.1 to 0.5 g/L of p-Naphtol. The Pb-free Sn—B plating solution may further contain 0.1 to 3 g/L of gelatin.

According to another aspect of the present invention, there is provided a plating method performed using a Pb-free Sn—B plating solution containing tin sulfate, which is a source of Sn ions, and dimethyl amine borane or trimethyl amine borane, which is a source of B ions.

The plating method may comprises plating the Pb-free Sn—B plating solution at a current density of 0.5 to 5 A/dm².

The plating method can be performed in room temperature.

ADVANTAGEOUS EFFECTS

According to the present invention, a Sn—B alloy plating layer that does not generate whisker can be provided.

Atoms of B are relatively small compared to atoms of Sn, and thus the atoms of B can penetrate into interstitial sites of Sn as will be described later. Accordingly, when a plating solution containing Sn and B forms a plating layer on a lead frame formed of Cu, Cu can be prevented from being diffused into the Sn, and thus generation of whisker on the plating layer is prevented. Consequently, electrical short circuits in a semiconductor lead frame having a plating region containing Sn and B including the Sn—B alloy plating layer is prevented, and durability of an electrical device using the semiconductor lead frame is improved.

A plating solution of Sn and B can generate a smooth plating surface. In the event of external shock, ductility of a smooth plating surface is better than a relatively rough plating surface. Also, a smooth plating surface can be transformed without any damage, and thus is suitable for providing a plating layer that can protect outer surface of an outer lead frame.

The plating solution of the present invention does not contain lead (Pb), and thus is harmless to the human body and is environmental friendly.

By using the plating solution of the present invention, the plating can be performed with relatively low current density at normal temperature, without separately heating the plating solution. Accordingly, productivity and profitability are improved.

BEST MODE

As described above, in a conventional Sn type plating layer, whisker on the surface of the plating layer is a problem. However, reasons for the generation of whisker are not clearly understood.

It has been noticed that when a plating layer formed of a Sn type plating solution is formed on a lead frame formed of Cu, diffusion speed of Cu is higher than that of Sn at the bounding interface between Sn and Cu.

In other words, since the diffusion speed of Cu is higher than the diffusion speed of Sn at the bonding interface between Sn and Cu, a Cu component of the lead frame diffuses toward a grain boundary of Sn of the plating layer. Then, an intermetallic compound is formed on the plating layer, having a composition of Cu₆Sn₅.

In the present application, it is considered that the intermetallic compound provides compressive stress inside Sn of the plating layer, and the compressive stress is resolved by the generation of whiskers in the Sn, which is a single crystal in a whisker form, on the surface of the plating layer.

Accordingly, the diffusion between metals is suppressed by inserting a metal having a small atom size into interstitial sites of a crystal structure of Sn, and thus the compressive stress inside Sn is reduced. Consequently, the generation of whisker is prevented. The metal having a small atom size may be boron (B).

However, the plating solution according to embodiments of the present invention does not include lead ions, and contains tin sulfate, which is a source of Sn ions, and dimethyl amine borane, which is a source of B ions.

The amount of tin sulfate (SnSO4), which is the source of Sn ions, may be 15 to 50 g/L, and the amount of dimethyl amine borane (DMAB), which is the source of B ions, may be 0.1 to 3.0 g/L. Trimethyl amine borane (TMAB) may be used as the source of B ions.

When the amount of the source of B ions is more than 0.1 g/L, the amount of B inserted into the interstitial sites of Sn of the plating layer is sufficient compared to a case when the amount of the source of B ions is less than 0.1 g/L. Accordingly as described above, an effect of suppressing the growth of the intermetallic compound of Sn and the basic material is remarkable, and thus whisker is not generated. Meanwhile, when the amount of the source of B exceeds 3.0 g/L, the amount of B inserted into the interstitial sites of Sn is saturated, and thus unnecessary expense is spent on B, which is uneconomical. Moreover, due to excessive amount of B, the surface of the plating layer becomes un-uniform, and a plating solution itself may be unstable.

30 to 70 ml/L of sulfuric acid (H₂SO₄) is added to the plating solution in order to control the conductivity of an electrolyte and mobility of ions such as Sn₂ ⁺. When the amount of sulfuric acid is less than 30 ml/L, the conductivity of the plating solution deteriorates, i.e. the electric resistance of the plating solution increases, and thus the plating speed decreases. Accordingly, productivity deteriorates, and the plating may be un-uniform. When the amount of sulfuric acid exceeds 70 ml/L, a significant amount of slime is generated in anode, and thus the plating solution becomes unstable, and the plating layer may be defective.

Also, 10 to 40 g/L of cresolsulfonic acid or phenolsulfonic acid of may be added to the plating solution in order to delay oxidation of Sn. When the amount of cresolsulfonic acid or phenolsulfonic acid is less than 10 g/L, Sn is easily oxidized, and when the amount of cresolsulfonic acid or phenolsulfonic acid exceeds 40 g/L, the plating solution may be unstable.

0.1 to 0.5 g/L of β-Naphtol may be added to the plating solution in order to control roughness of the surface of the plating layer. When the amount of β-Naphtol is less than 0.1 g/L, coarse crystal particles may be generated, and when the amount of β-Naphtol exceeds 0.5 g/L, the surface of the plating layer may be very rough.

In addition, 0.1 to 3 g/L gelatin may be added to the plating solution. When the amount of gelatin is less than 0.1 g/L, the crystal particles are too coarse, and when the amount of gelatin exceeds 3 g/L, a lot of needle shapes or protrusions may be generated.

The plating solution is electroplated on a Cu plate, which may be the main material of the lead frame. The Cu plate is used as a cathode and soluble Sn is used as an anode. The current density during the plating may be 0.5 to 5 A/dm², and preferably 1 to 3 A/dm². In the following embodiment, the current density is 1 A/dm². When the current density exceeds 5 A/dm2, the plating surface is very rough, the crystal growth is un-uniform, and the plating is unstable. Accordingly, reliability of the plated film deteriorates. When the current density is less than 0.5 A/dm², the plating time is too long, thereby adversely affecting productivity.

The plating temperature of the plating solution is room temperature (25±3° C.). When the temperature of the plating solution is increased, such as to 50±3° C., the additives may be resolved, and thus the plating may be abnormally performed. As a result, whisker may be generated.

MODES OF THE INVENTION

Hereinafter, the present invention will be described more fully with reference to the following examples.

Embodiment 1

A plating solution containing 15 g/L of tin sulfate, 30 ml/L of H₂SO₄, 10 g/L of cresolsulfonic acid, 0.1 g/L of β-Naphtol, and 0.1 g/L of gelatin is produced.

In Experiment 1, 0.1 g/L of DMAB is further added to the plating solution, in Experiment 2, 0.5 g/L of DMAB is further added to the plating solution, and in Experiment 3, 3 g/L of DMAB is further added to the plating solution.

Plating is performed under the same plating conditions described above. In other words, a Cu plate is used as a cathode, soluble Sn is used as an anode, the current density is 1 A/dm², and the plating temperature is normal temperature.

The plating layers of Experiments 1 through 3 are stored at room temperature for 12 months, and then it is determined whether whisker is generated on the surface of the plating layers.

FIGS. 1 (a) through (c) are scanning electron microscope (SEM) photographic images respectively illustrating surface states of the plating layers in Experiments 1 through 3 after the room temperature storage test.

Embodiment 2

A plating solution containing 30 g/L of tin sulfate, 50 ml/L of H₂SO₄, 20 g/L of cresolsulfonic acid, 0.3 g/L of β-Naphtol, and 0.5 g/L of gelatin is produced.

In Experiment 4, 0.1 g/L of DMAB is further added to the plating solution, in Experiment 5, 0.5 g/L of DMAB is further added to the plating solution, and in Experiment 6, 3 g/L of DMAB is further added to the plating solution.

Plating is performed under the same plating conditions of Example 1. The is plating layers of Experiments 4 through 6 are stored at room temperature for 12 months, and then it is determined whether whisker is generated on the surface of the plating layers.

FIGS. 2 (a) through (c) are SEM photographic images respectively illustrating surface states of the plating layers in Experiments 4 through 6 after the room temperature storage test.

Embodiment 3

A plating solution containing 50 g/L of tin sulfate, 70 ml/L of H₂SO₄, 40 g/L of cresolsulfonic acid, 0.5 g/L of β-Naphtol, and 1.0 g/L of gelatin is produced.

In Experiment 7, 0.1 g/L of DMAB is further added to the plating solution, in Experiment 8, 0.5 g/L of DMAB is further added to the plating solution, and in Experiment 9, 3 g/L of DMAB is further added to the plating solution.

Plating is performed under the same plating conditions of Example 1. The plating layers of Experiments 7 through 9 are stored at room temperature for 12 months, and then it is determined whether whisker is generated on the surface of the plating layers.

FIGS. 3 (a) through (c) are SEM photographic images respectively illustrating surface states of the plating layers in Experiments 7 through 9 after the room temperature storage test.

Embodiment 4

A plating solution containing 50 g/L of tin sulfate, 70 ml/L of H₂SO₄, 40 g/L of cresolsulfonic acid, 0.5 g/L of β-Naphtol, and 3.0 g/L of gelatin is produced.

In Experiment 10, 0.1 g/L of DMAB is further added to the plating solution, in Experiment 11, 0.5 g/L of DMAB is further added to the plating solution, and in Experiment 12, 3 g/L of DMAB is further added to the plating solution.

Plating is performed under the same plating conditions of Example 1. The plating layers of Experiments 10 through 12 are stored at room temperature for 12 months, and then it is determined whether whisker is generated on the surface of the plating layers.

FIGS. 4 (a) through (c) are SEM photographic images respectively illustrating surface states of the plating layers in Experiments 10 through 12 after the room temperature storage test.

Embodiment 5

A plating solution containing 30 g/L of tin sulfate, 50 ml/L of H₂SO₄, 20 g/L of cresolsulfonic acid, 0.5 g/L of β-Naphtol, and 3.0 g/L of gelatin is produced.

In Experiment 13, 0.1 g/L of DMAB is further added to the plating solution, in Experiment 14, 0.5 g/L of DMAB is further added to the plating solution, and in Experiment 15, and 3 g/L of DMAB is further added to the plating solution.

Plating is performed under the same plating conditions of Example 1. The plating layers of Experiments 13 through 15 are stored at room temperature for 12 months, and then it is determined whether whisker is generated on the surface of the plating layers.

FIGS. 5 (a) through (c) are SEM photographic images respectively illustrating surface states of the plating layers in Experiments 13 through 15 after the room temperature storage test.

Embodiment 6

A plating solution containing 50 g/L of tin sulfate, 70 ml/L of H₂SO₄, 40 g/L of cresolsulfonic acid, 0.5 g/L of β-Naphtol, and 1.0 g/L of gelatin is produced.

In Experiment 16, 30 ppm of DMAB is further added to the plating solution and in experiment 17, 4 g/L of DMAB is further added to the plating solution.

Plating is performed under the same plating conditions of Example 1. The plating layers of Experiments 16 and 17 are stored at room temperature for 12 months, and then it is determined whether whisker is generated on the surface of the plating layers.

FIG. 6 (a) is a SEM photographic image of the plating layer after plating the surface of the plating layers in Experiment 16, FIG. 6 (b) is a SEM photographic image of the plating layer of Experiment 16 after a room temperature storage test, and FIG. 6 (c) is an SEM photographic image illustrating surface states of the plating layer in Experiment 17 after the room temperature storage test.

As illustrated in FIGS. 1 (a) through 5 (c), whiskers are not generated on the surface of the plating layers according to Embodiments 1 through 5 even after a long time.

Also, as illustrated in FIGS. 6 (a) and 6 (b), when the amount of DMAB is extremely low, whiskers are not generated right after the plating layer is manufactured, but are generated after a long time has elapsed.

As illustrated in FIG. 6 (c), when the amount of DMAB is large, whiskers are not generated, but the surface of the plating layer is un-uniform and rough.

COMPARATIVE EXAMPLE

In Comparative Embodiments 1 through 5, DMAB is excluded respectively from Embodiments 1 through 5.

Plating is performed under the same plating conditions as Embodiment 1, and the plating layers of Comparative Embodiments 1 through 5 are stored at room temperature for 12 months and then checked for whiskers.

FIGS. 7 (a) through (e) are SEM photographic images respectively illustrating surface states of the plating layers in Comparative Experiments 1 through 5 after the room temperature storage test.

As illustrated in FIGS. 7 (a) through (e), when the plating is performed using a plating bath without a source of B ions, whiskers are generated on the surface of the plating layers.

Accordingly, the present invention provides a Pb-free plating layer that can suppress generation of whiskers.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims. 

1. A Pb-free Sn—B plating solution containing tin sulfate, which is a source of Sn ions, and dimethyl amine borane or trimethyl amine borane, which is a source of B ions.
 2. The Pb-free Sn—B plating solution of claim 1, wherein the amount of the source of Sn ions is 15 to 50 g/L.
 3. The Pb-free Sn—B plating solution of claim 1 wherein the amount of B ions is 0.1 to 3.0 g/L.
 4. The Pb-free Sn—B plating solution of claim 1, further containing 30 to 70 mL of sulfuric acid.
 5. The Pb-free Sn—B plating solution of claim 1, further containing 10 to 40 g/L of cresolsulfonic acid or phenolsulfonic acid.
 6. The Pb-free Sn—B plating solution of claim 1, further containing 0.1 to 0.5 g/L of D-Naphtol.
 7. The Pb-free Sn—B plating solution of claim 1, further containing 0.1 to 3 g/L of gelatin.
 8. A plating method performed using a Pb-free Sn—B plating solution containing tin sulfate, which is a source of Sn ions, and dimethyl amine borane or trimethyl amine borane, which is a source of B ions.
 9. The plating method of claim 8, comprising plating the Pb-free Sn—B plating solution at a current density of 0.5 to 5 A/dm².
 10. The plating method of claim 8, wherein the amount of the source of Sn ions is 15 to 50 g/L and the amount of B ions is 0.1 to 3.0 g/L, and the Sn—B plating solution further contains 30 to 70 ml/L of sulfuric acid, 10 to 40 g/L of cresolsulfonic acid or phenolsulfonic acid, 0.1 to 0.5 g/L of β-Naphtol, and 0.1 to 3 g/L of gelatin.
 11. The plating method of claim 9, wherein the amount of the source of Sn ions is 15 to 50 g/L and the amount of B ions is 0.1 to 3.0 g/L, and the Sn—B plating solution further contains 30 to 70 ml/L of sulfuric acid, 10 to 40 g/L of cresolsulfonic acid or phenolsulfonic acid, 0.1 to 0.5 g/L of β-Naphtol, and 0.1 to 3 g/L of gelatin. 